I think the lowest I took my PLUQ (sewn up version) was about 45°F. Before I found under quilts, I used the blue closed-cell foam pads from WalMart. The lowest I attempted pads alone was 15°F and I about froze myself into a hypothermic coma (I only had one layer of padding).
Another time, I attempted to tape a space blanket (mylar) to my Grand Trunk Ultralight hammock. I then used my Kelty Galatic 35°F bag as a pull-up bag around my hammock. I think that combination got me into the 20s°F.
You can see in this photo the Mylar sticking out of the end, if you look closely.
I would guess if you combined all three (PLUQ, CCF pad, and Mylar space blanket) you could get into the 20s°F with a good top quilt, maybe lower.
I'm suppose to have some cold nights (15°F) this week, so maybe I'll test a PLUQ and CCF combo just to try it out.
But, yes -- test yourself and see It's kind of fun, especially if you can run back inside if it gets too cold. My wife likes to laugh at me when I do that.